Cabinet and door construction for microwave heating appliances



K. FRITZ 3,436,508

CABINET AND DOOR CONSTRUCTION FOR MICROWAVE HEATING APPLIANCES April 1,1969 Filed Aug. 23, 1966 FIG.

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INVENTOR. K1424 fiP/rz I ATTO/PA/[A United States Patent US. Cl.21910.55 6 Claims ABSTRACT OF THE DISCLOSURE The door is formed with aprojection, and the cabinet, in the region surrounding the door, has alip which is spaced to form a wave guide of a characteristic impedancemuch lower than the resistance of the space within the cabinet, todetect still leaking radiation from the door, a radiation pickup loop,in the form of a conductor, is located on the cabinet just beyond theouter edge of the door opening, surrounding the door opening. Forpositive seating and minimum leaking, the door is hinged to the cabinetby a linkage which permits swinging of the door for a major extend, andperpendicular motion of the door with respect to the cabinet just beforeclosing.

The present invention relates to microwave ovens, and more particularlyto a cabinet and door construction for microwave ovens.

Heating by microwaves is particularly suitable to defrost frozen foodand to obtain quick and uniform heating throughout the food. Microwaveovens are, however, expensive; the frequency used is required to behigh, in the order of 20 cm. or less and the frequency band width mustbe small. Lowering the frequency could effect economy, but it is thendifficult to provide a radiation tight door closure.

Frequencies having a wavelength in the order of 30 to 40 cm. have theadvantage that the surface layer of the food within the cabinet ispenetrated better, apart from the fact that the cost of the apparatus islower. If the wave length of the radiation Within the cabinet is,however, of about the same order of magnitude as a dimension of thespace itself, the first harmonic will be the primary radiation therein.It is diflicult to manufacture doors, particularly in mass production,which seal such longer wave radiation to the inside of the cabinet andeffectively prevent escape of any radiation to the outside. The amountof permissible stray radiation is very small, without exceeding safetyand interference limits; it may be only as little as watts. These safetyand manufacturing difliculties have heretofore prevented acceptance ofmicrowave ovens utilizing such higher frequencies.

It is an object of the present invention to provide a cabinet and doorconstruction which meets the above requirements, which can bemanufactured in mass production processes, and can be hunginexpensively.

In spite of precautions taken to provide a radiation seal to theoutside, it is possible that radiation in excess of that permitted canescape. According to a feature of the invention, a radiation monitoringarrangement may readily be combined with the door construction to givean alarm when the radiation leaking to the outside of the cabinetexceeds a predetermined and safe limit.

Briefly, in accordance with the present invention, the cabinet is formedwith a lip surrounding the door opening, which lip extends inwardly,preferably in the form of a truncated cone or pyramid, like a funnel;the door fits into the truncated funnel, with matching sides, spaced avery small distance apart. The spacing may be determined by 3,436,508Patented Apr. 1, 1969 a plastic coating, applied to the door or to thecabinet lip. A metallic contact is provided by the door against thecabinet on the outside, that is beyond the termination of the parallelsides of door projection and lip.

The door projection and the lip together form a wave guide having acharacteristic impedance to 1,000 times, that is 2 to 3 orders ofmagnitude (on a decimal scale) lower than the internal impedance of thecabinet. Thus, a discontinuity of impedance is formed within the cabinetin the region of the juncture of door projection and lip, whichdiscontinuity will cause reflection, rather than penetration into thelow impedance wave guide. Such penetration as does occur will be of suchlow power that the metallic contact at the outside of the wave guide,between door and cabinet, can readily short circuit the stilltransmitted radiation.

Without providing the wave guides, the danger may exist that if thecontact between frame and door is not absolutely perfect, hot spots maydevelop, including possibly points causing arcing over due to energytransmitted to the points of poor contact. By reflecting the energywithin the cabinet away from the metallic contact, the energy at themetallic contact will be low and slight imperfections or misalignmentarising in manufacture, or in use, will not cause harmful results orleakage of radiation.

In order to provide for location of the door within the lip with thesame alignment, time after time, the door is suspended by means of alinkage arrangement, for example a parallelogram arrangement to providefor parallel seating at the final point of travel of the door within thelip.

The polarization of waves leaving the interior of the cabinet isnormally approximately vertical to the surface of the door or its frame.It is therefore simple to provide an antenna, such as a wire or a numberof wires looped around the door, mounted in an insulating material, suchas a plastic head or strip, to pick up radiation leaking beyond theseal. These monitoring wires, acting like antennas, are then connectedto an alarm device, for example over an amplifier. The alarm device cangive an audible, visual, or other signal to warn of excessive radiationbeyond the cabinet, and further may actuate a safety switch todisconnect the radiation generator and prevent damage or hazards.

The spacing between the internally projecting lip surrounding the dooropening, and the matching projecting portions of the door itself, may bein the order of 1 mm. or so; if a plastic liner is used, either on thedoor or on the lip, the resistance of the wave guide formed by door andlip will be in the order of about 1 ohm or less. The wave resistance ofthe cabinet iself may vary from about 100 ohms to an average of severalhundred ohms to in excess of thousand ohms. Thus, severe discontinuitiesof resistance will occur just at the point where reflections aredesired, and little radiation will escape in the space formed by thewave guide between door and cabinet.

The structure, organization and operation of the invention will now bedescribed more specifically in the following detailed description withreference to the accompanying drawings, in which:

FIG. 1 is a cross sectional view illustrating the door construction, andadjacent parts of the cabinet, in section;

FIG. 2 is a view similar to FIG. 1, showing an alternative embodiment ofdoor construction, with the door partly open;

FIG. 3 is a pictorial view of a cabinet with a door of a constructionsimilar to FIG. 1; and

FIG. 4 is a schematic circuit diagram of the control circuit.

The cabinet 1, having a space 1 therein, which space is intended to besubjected to microwave energy, is closed 011 by a door having a bottomor base plate 2 and side flanges 2'. The base of the door 2 isprojecting from the side flanges 2, by having inwardly tapering orslanting walls 2" so that the entire construction is in the form of atruncated, closed funnel. Outwardly of the projection 2, the flange 2'is formed with an inwardly extending projection 3, extending on allsides of the flange 2. Projection 3 comes to a point 4. When the door isseated within the cabinet, as shown in FIG. 1, projection 3 with itspoint 4 will bear against a seal 5, shown as a plastic seal withmetallized surface, or covered with flexible wire netting, and furtheron the side against a metal strip '8, connected to the housing 1 of thecabinet. Strip 8 may be a single unit, or formed with a spring overlayto make good electrical contact with projection 3.

The door opening of cabinet 1 matches the truncated funnel shape of thedoor. In particular, a lip, formed by a pair of slanting walls 6, 6surrounds the door opening; between the walls 6, 6, which may be formedby folded material so as to present a re-entrant fold, insulatingmaterial 7 may be located which may be a dissipating substance.

The door is supported on door hinges; a bracket 22 (FIG. 1, FIG. 3)extends from the cabinet to hold the hinge mechanism. The link 20,extending generally perpendicularly from base 2 of the door, and hingedat 10 thereto, is hinged at 11 to a second link 21, again extendingperpendicular to link 20. Link 21 is hinged at 12 to bracket 22. Bysuitable design of the length of brackets 22 and link 20, a toggleaction can be obtained so that the door will snap shut into the dooropening formed by reentrant walls 6, 6'. A spring 24 is furtherprovided, shown connected with hinge 12 in order to keep the door in aclosed position. A cover plate 14 closes off the mechanism, to which ahandle may be attached. FIG. 3 shows the door construction with coverplate 14 and handle 15 removed.

FIG. 2 shows an alternative form of door holding construction, using aparallelogram arrangement, having pivot points 30, and 31, 31, hinged at33 to a link 35 which, in turn, is hinged at 32 to the door bracket 22.A spring 34 again tends to hold the door in closed position. Again, bysuitable choice of the length of the links and of the bracket,respectively, the arrangement can function as a toggle to snap shut.FIG. 2 also shows an embodiment in which the door opening is defined bya single wall '6, rather than by the double, re-entrant constructionincluding walls 6, 6' of FIG. 1.

The sidewalls 2" of the door and wall 6 (or 6) of the cabinet areseparated by a small distance, for example in the order of 1 mm. or so.This separation can be obtained, for example by covering either the doorsidewalls 2", or the cabinet door opening wall 6 (or 6') by a thinplastic liner, shown at 16 in FIG. 2 (and omitted from FIG. 1 forpurposes of clarity). "Separating walls 2" and 6, rather than providinga short-circuited electrical contact, will form between walls 2" and 6 awave guide having a characteristic impedance very much less than thecharacteristic impedance of the interior of the cabinet. A severediscontinuity will thus result at the edge of the thus formed wave guideindicated at D (FIG. 1) causing reflections of waves internal of thecabinet and preventing transfer of energy to the region between door andcabinet. Any remainnig energy that is not reflected will be conductedalong the wave guide. The lvel of this energy will be so low that it canbe short-circuited easily and without danger of arcing or flash-over atthe contact between metal strip 8 and projection 3; or between contactsprings conductively connected, for example riveted to metal bracket 8,so as to form a good and resilient contact with projection 3. Theparticular hinge arrangement shown in FIG. 1, or the parallelogramarrangement of FIG. 2 (which requires more hinge points than FIG. 1) isprovided in order that the door sides 2" may seat parallel to thematching walls of the cabinet. Bracket 13 (FIG. 1) and cover plate 14limit the side excursion of links 20,

21 and thus provide for limited parallel motion at the final travel ofthe door towards their seating position; while unrestricted hinge 12(FIG. 1) and 32 (FIG. 2) permits unlimited swinging movement to open thedoor to any desired position. A mere hinging arrangement of the door isnot desirable, however, in order to prevent damage to the side wallsupon final seating and to effect parallel seating of sidewalls 2" withthe interior walls 6, or 6, respectively, of the cabinet.

It is possible that a small amount of radiation may still escape. Inorder to detect possible leakage of radiation larger than thatpermissible, a simple and effective detection arrangement isincorporated in the construction according to the present invention.Plastic bead 9 contains a pair of wires 9, looped around the dooropening, which will act as an antenna and pick up any radiation notbypassed or completely short-circuited by contact of metal part 8against projection 3. Radiation so picked up can 'be amplified in anamplifier 29 (FIG. 4) which, when the permissible limit of radiationreceived by wires 9 exceeds a certain limit, can be arranged to give analarm, for example by triggering an audible or visual indicator 19 (FIG.3, FIG. 4) and simultaneously operating a relay to disconnect power tothe input of the microwave generator supplying energy to the cabinet.

The walls 6 (or 6) of the cabinet form a lip extending around the dooropening, converging at an angle to match the angle of convergence of thesum of the funnel shaped projecting section of the door. The dielectric7 within the re-entrant portion 6, 6' can act as a dissipating sectionto dissipate radiation along the wave guide formed therebetween, for anyradiation not reflected at point D and reaching point E within the doorand the cabinet structure. In most instances, however, a single Wall 6,to which a dielectric such as plastic coating 16 is applied, both forspacing as well as for mechanical sealing purposes, will suflice. Theenergy reaching point B is usually so low that it can readily beshort-circuited by contact between projection 3 and cabinet contactmember 8.

I claim:

1. In a microwave heating appliance, a cabinet and door constructionarranged to prevent escape of radiation from within the cabinet, saidcabinet and door construction comprising a lip formed on the cabinetsurrounding the door opening and extending inwardly of the cabinet; aprojection formed on said door, extending parallel to said lip andspaced closely therefrom when said door is closed, said lip and saidprojection jointly forming a wave guide having a characteristicresistance which is to 1,000 times lower than the resistance of thespace within the cabinet, and forming an electrical contact between thedoor and the cabinet adjacent said door opening and substantially at theouter end of the thus formed wave guide, the abrupt transition ofimpedance from within the cabinet to the impedance of the thus formedwave guide causing reflections and preventing escape of radiationoutside of said cabinet in the region of said door within said dooropening; and radiation pickup means located outside of said cabinet andadjacent said electrical contact forming means to detect radiationleaking from within said cabinet and not bypassed by said contactforming means.

2. A construction as claimed in claim 1 wherein said radiation pickupmeans includes a loop of conductors located on the cabinet just beyondthe outer edge of the door opening and surrounding said door opening.

3. A construction as claimed in claim 1 including alarm means connectedto said radiation pickup means, said alarm means being operative upondetection of radiation in excess of a predetermined level by saidradiation pickup means.

4. A construction as claimed in claim 1 including support and operatingmeans to interconnect said door and said cabinet, said support andoperating means including a linkage means permitting swinging movementof said door with respect to said cabinet for a major extent andmovement of said door perpendicular to said cabinet just before closing,so that said lip and said projection will seat in parallel.

5. A construction as claimed in claim 4 wherein a first link is providedextending generally perpendicular to the door and mounted for limitedswinging movement thereon; a second link extending generally parallel tosaid door and mounted for limited swinging movement with respect to saidfirst link at one end thereof, and secured for swinging movement at theother end thereof to said cabinet.

6. A construction as claimed in claim 4 wherein said linkage means are aparallelogram attachment secured to said door at one fixed parallelside; and holding links secured to the other of said parallel sides andinterconnecting said other parallel side and said cabinet.

6 References Cited UNITED STATES PATENTS 6/1950 Proctor et al. 21910.675/1965 Ironfield 21910.55 7/1965 De Vries et a1 2191().55 11/1965McAdams 2191055 X FOREIGN PATENTS 4/1947 Great Britain.

RICHARD M. WOOD, Primary Examiner. L. H. BENDER, Asistant Examiner.

U.S. Cl. X.R.

